Modern portable radios and other electronic equipment require filters of very compact form and good electrical properties. Surface acoustic wave (SAW) filters are especially desirable for such purposes.
SAW filters are known in the prior art. For example, an article by Meirion Lewis entitled, "Saw Filters Employing Interdigitated Interdigital Transducers, IIDT", Ultrasonics Symposium, 1982, pages 12-17, describes SAW filters using interdigital transducers of interdigital electrodes on piezo-electric materials, each transducer having the same number of electrode fingers. Further, U.S. Pat. No. 4,492,940 to Hikita describes an acoustic surface wave bandpass filter using a linear array of alternate I/O transducers arranged in sequence, each having multiple interdigital electrode fingers, where the number of fingers in the input transducers and the number of fingers in the output transducers decreases from the center to the end of the array. Additionally, U.S. Pat. No. 4,649,357 to Nagai et al., describes a surface acoustic wave filter using a linear array of alternate I/O transducers arranged in sequence, each having multiple interdigital electrode fingers, where the distances (along the array) between the centerlines of adjacent transducers differ by an integral multiple (0, .+-.1, .+-.2, etc.) of wavelengths, and U.S. Pat. No. 4,931,755 to Sakamoto et al., describes use of an auxiliary capacitive transducer or other shunt capacitance to adjust the filter cut-off above the pass-band. The above-listed patents are incorporated herein by reference.
Despite the considerable effort by many researchers, numerous problems remain in connection with SAW filters, as for example, setting the optimum input/output (I/O) impedance levels, optimizing the spectral response, reducing internal reflections within the filter, and simplifying the fabrication process. These features of SAW filters affect their utility and performance in electronic apparatus. Hence, there continues to be an ongoing need for improved filters and filter applications.
As used herein the abbreviation SAW is intended to stand for "surface acoustic wave". As used herein the word "filter", singular or plural, is intended to include any element having a frequency dependent transfer function, and the words "SAW filter", singular or plural, are intended to refer to elements employing surface acoustic waves and having a frequency dependent transfer function.